1. Field of the Invention
The present invention generally relates to a reverse rotation control system for a two-cycle engine which makes it possible to drive or run a motor vehicle equipped with the two-cycle engine exchangeably in forward or rearward direction by reversing the rotation of the engine. More particularly, the invention is concerned with a reverse rotation control system for a two-cycle engine mounted on a motor vehicle, which system can realize a reverse rotation control with inexpensive hardware structure without incurring degradation with regard to exhaust gas composition and ignition performance of the motor vehicle equipped with the two-cycle engine.
2. Description of Related Art
In general, a four-cycle engine mounted on a motor vehicle such as a passenger car is equipped with a clutch and a gear box at the output side of the engine for deriving the output power thereof.
However, in the case of small-size motor vehicles for specific purposes such as snowmobiles, all-terrain vehicles and the like, a two-cycle engine of an inexpensive structure is mounted. In this conjunction, it is further noted that in these motor vehicles, the space for installing or accommodating the engine is limited.
Such being the circumstances, no gear box is ordinarily installed as the reverse rotation control system for the two-cycle engine in these types of the motor vehicles, wherein the output torque of the engine is derived through the medium of only a centrifugal-type automatic transmission implemented in the form of a V-belt transmission.
Consequently, the motor vehicle can be driven only in the forward direction. Thus, manpower is required for moving the motor vehicle backwards or rearwards as in the case where the motor vehicle is to be taken out from a garage or it is to be disburden from a carrier such as a lorry, giving rise to a problem that the motor vehicle is very inconvenient to handle.
For evading the problem mentioned above, it has heretofore been proposed that a clutch and a gear box are provided also for the motor vehicle equipped with the two-cycle engine by affording a sufficient space for installation of the two-cycle engine so that the traveling direction of the motor vehicle can be changed over between the forward direction and the backward or rearward direction by manipulating a gear change lever, as in the case of the four-cycle engine.
For having a better understanding of the concept underlying the present invention, a hitherto-known or conventional reverse rotation control system for a two-cycle engine of a motor vehicle will be reviewed in some detail.
FIG. 4 is a block diagram showing schematically and generally a configuration of a prior art reverse rotation control system for a two-cycle engine of a motor vehicle in which a conventional gear box is employed. Referring to FIG. 4, an internal combustion engine (hereinafter referred to simply as the engine) 1 driven in two cycles is installed on a motor vehicle (not shown). The engine 1 has an output shaft 2 which rotates in one direction as indicated by an arrow, wherein a driving torque generated by the engine 1 is outputted through the medium of a clutch 3 and a gear box 4. The gear box 4 is provided with a back gear train for allowing the motor vehicle to be driven rearwards.
Furthermore, a change lever 5 is provided in the gear box 4 for allowing a driver to manually change over gear trains. A rotation sensor 6 for detecting the engine speed (rpm) as well the as angular position of a crank shaft (crank angle) of the engine is implemented in the form of an electromagnetic pickup device or the like and provided in association with the output shaft of the engine 1. A rotation signal SG derived from the output of the rotation sensor 6 is inputted to an ignition control unit 10 which may be constituted by a microprocessor or microcomputer.
The ignition control unit 10 is so designed or programmed as to arithmetically determine control timings for the engine 1 for issuing an ignition signal P on the basis of operation state information which can be derived from not only the rotation signal SG mentioned above but also other signals outputted from other various sensors (not shown).
An ignition coil 11 is realized in the form of a transformer having a primary winding and a secondary winding for generating in response to the ignition signal P a secondary voltage boosted up upon interruption of the primary current, whereby a high voltage for firing is applied to a spark plug 12 of the engine 1. In this conjunction, it is to be noted that the engine 1 is subjected to rotation control in a predetermined direction by controlling the ignition timing on the basis of the rotation signal SG.
In the two-cycle engine of a motor vehicle equipped with the conventional reverse rotation control system such as shown in FIG. 4, the rotation output or output torque of the engine 1 can be reduced as desired through the gear box 4 while the driving direction of the motor vehicle can be changed over between the forward direction and the rearward direction with the aid of the back gear train.
However, with the arrangement shown in FIG. 4, difficulty will be encountered in assuring a space around the engine 1 for affording accommodation and installation of the gear box 4. In particular, in the case of the snowmobile and the all-terrain vehicle mentioned previously, it is difficult to make available an engine room for accommodating therein the engine 1 itself. Consequently, additional provision of the gear box 4 will incur remarkable increase in the manufacturing cost of the motor vehicle.
At this juncture, it should be mentioned that the two-cycle engine has such a feature that the crank shaft can be rotated in any one of the forward or reverse direction by selectively controlling the ignition timing, differing from the four-cycle engine.
In actuality, a reverse rotation control system for a two-cycle engine has been realized by making use of the above-mentioned feature. By way of example, there is disclosed in U.S. Pat. No. 5,036,802 issued in 1991 such a reverse rotation control system for a two-cycle engine of a motor vehicle which makes it possible to drive the motor vehicle either in the forward direction or the rearward direction through the reverse rotation control of the engine 1 by using a centrifugal-type automatic transmission (not shown) without resorting to the use of the gear box 4.
In this case, when a driving direction of a motor vehicle equipped with a two-cycle engine is to be reversed, a driver manipulates a rotation reversing lever in a normal rotation state of the engine 1 (corresponding to e.g. forward traveling of the motor vehicle). Then, the ignition control unit 10 lowers the rotation speed (rpm) of the engine 1 by forcibly causing misfire to take place in the engine 1. When the engine rotation speed has thus been lowered to a second predetermined engine rotation speed (e.g. 500 rpm) which is suited for the reverse rotation control (i.e., control for reversing the rotating direction of the engine), the ignition timing at which the ignition signal P is applied is caused to overadvance beyond a normal advance control position (lying within a range of 5.degree. to 30.degree. before the top dead center TDC in terms of crank angle, i.e., BTDC 5.degree. to 30.degree.). With the overadvance control for the ignition timing described above, the ignition timing is set, for example, at BTDC 40.degree. (i.e., at the crank angle of 40.degree. before the top dead center or BTDC 40.degree.), to thereby allow the engine 1 to transit from the normal rotation state (corresponding to e.g. forward running of the motor vehicle) to the reverse rotation state (corresponding to e.g. rearward traveling direction of the motor vehicle).
Thereafter, the ignition control unit 10 regards the reverse rotation direction as the normal rotation direction and the ignition signal P is generated at the ordinary ignition timing for sustaining continuously the reverse rotation state of the engine 1. Thus, the motor vehicle can be driven rearwards. Parenthetically, when the engine 1 is to be restored from the reverse rotation state to the normal rotation state, the control process similar to that described above is carried out by regarding the current rotating direction of the engine (i.e., the reverse rotation) as the normal rotating direction.
With the reverse rotation control system described above, there arises the necessity for monitoring or detecting constantly the rotation number of the engine 1 in order to decide that the engine rotation number has actually decreased to the second predetermined engine rotation number which permits the reverse rotation by comparing the detected engine rotation number with the second predetermined engine rotation number, as a result of which not only the cost of the engine system but also the burden imposed on the ignition control unit 10 increases.
Certainly, the manufacturing cost of the motor vehicle can be reduced because the gear box 4 can be spared. However, because the engine rotation number or engine speed (rpm) is lowered to the second predetermined engine rotation number at which the rotation of the engine 1 can be reversed by resorting to the misfire control technique described above, unburnt gas is discharged from the engine 1 during the misfire control process, giving rise to a problem. Furthermore, during the misfire control process, deposition of fuel components on a discharge electrode of the spark plug 12 is likely to occur, as a result of which ignition performance of the engine 1 will be degraded at a succeeding ignition timing, to a disadvantage.
As can be understood from the foregoing, the conventional reverse rotation control system for the two-cycle engine for a motor vehicle suffers a problem that when the gear box 4 such as shown in FIG. 4 is employed, there arises the necessity for ensuring a space for installation of the gear box 4 around the engine 1, which will of course lead to increasing of the cost.
On the other hand, in the reverse rotation control system for the engine 1 in which the engine rotation number is once lowered and then the ignition timing is overadvanced, as is disclosed in U.S. Pat. No. 5,036,802, there arises the necessity for monitoring the engine rotation speed (rpm) up to a time point immediately before the start of the overadvanced ignition timing control in order to decide that the engine rotation number has decreased to the second predetermined engine rotation number suited for the rotation reversing control. Thus, the burden imposed on the ignition control unit 10 increases, being accompanied with increase of the cost of the motor vehicle. Additionally, the quality of exhaust gas composition and the firing performance will be degraded during the misfire control process, giving rise to another problem.